A. Field of the Invention
This invention relates generally to the field of servo follower proportional control spool valves.
B. Background Art
Servo follower proportional control valves are well known in the art and are generally described in 1980-81 Fluid Power Handbook and Directory, page A-141 and in National Conference on Fluid Power, 1976 Electro-proportional Position Controls--An Analysis for Application on Various Hydraulic Control Functions by D. W. Swaim.
Prior proportional control valves have left much to be desired with respect to rapid response to input commands which may be rapidly changing. In prior spool valves, such rapid response was adversely affected by the use of dynamic seals. The seals would enter the space between the bore and the active valve element and thus increase friction. In this way, such seals were known to cause breakaway and running friction between the valve element and bore, thereby decreasing the ability to rapidly respond as well as decreasing ion frequency tracking ability.
Another objectionable feature of prior proportional control valves decreasing rapid response has been the relatively large driving chamber volume. The large chambers required a relatively large amount of fluid to produce movement, which in turn required a substantial ammount of time. For example, see U. S. Pat. Nos. 2,526,709; 2,555,755; and 4,085,920.
Many prior proportional valves are of the electrohydraulic type which have flapper nozzles and which produce valve spool movement. The following are representative patents directed to such flapper nozzle valves: 3,874,405; 3,598,151; 3,598,152; 3,742,980; 3,457,956; and 3,749,128. In these systems a DC electrical signal to the coils of the pilot stage of a force-balanced torque motor develops a torque moving the armature-flapper either clockwise or counter clockwise between the nozzles. This movement restricts flow out of one of the nozzles and eases flow out of the other with the pressure unbalance driving the spool. These flapper-nozzle proportional valves have left much to be desired as a result of the constant leakage of the valve including the time when the valve is at null. This leakage at null is a serious disadvantage since the valve may be at null for a long period of time and would lose a substantial amount of energy, as for example a quarter to four-tenths of a gallon per minute per valve. For multiple valves the loss has been considerable and has required cooling members to remove the lost energy. Another problem with flapper-nozzle proportional valves has been in their small orifices to control the flapper as well as small spring feedback. The small orifices are liable to clogging and the small springs are liable to fatigue. Spool proportional valves suffer from similar problems as they also have small orifices and high null leakage. In such valves the leakage at null undesirably increases as flow increases. See U.S. Pat. No. 3,827,453. The following are examples of patents on spool within spool valves: U.S. Pat. Nos. 3,027,880, 2,748,752; 3,530,895; 3,163,179; 4,114,650; 3,013,539; 3,459,224; and 4,046,059.